1. Field of the Invention
This invention relates to phosphor pastes for printing capable of providing a uniform and high-resolution phosphor coating which is required for the formation of a fluorescent screen of a cathode ray tube (hereinafter abbreviated as "CRT").
2. Description of the Prior Art
A CRT typified by a TV Braun tube is a display device in which electron beams emitted from an electron gun are projected on a phosphor coating to excite the phosphor to produce luminous spots in the form of a pattern. A variety of visual equipment making use of a high electronic technique has been developed. Their display devices range from color display devices to monochrome display devices. These display devices also vary widely in size from large to very small. It is the phosphor coating formed on a glass plate of a CRT that governs the performance of the CRT which makes up the heart of such visual equipment. As production processes for the phosphor coating, there have been known, for example, slurry processes, settling processes, electrophoretic deposition processes and vacuum evaporation processes for a monochrome CRT. Among these, the settling process making use of a phosphor, and an inorganic binder is generally used to form a phosphor coating for a high-resolution monitor CRT. However, the surface of the phosphor coating obtained by this process is excessively rough. This process can therefore hardly provide a phosphor coating of uniform thickness and high resolution. On the other hand, the phosphor coating of a color CRT contains three phosphors of red, green and blue colors arranged in the form of dots or stripes. Photo-curing processes using a shadow mask have conventionally been used for its formation. Use of a shadow mask with a fine pattern formed therein is however indispensable for the formation of the fluorescent screen of a color CRT by this process. A shadow mask of higher accuracy is required as the CRT becomes smaller in size or a CRT having a screen of higher resolution is desired. In addition, it is also necessary to develop a high-performance phosphor slurry usable for the formation of such a phosphor coating. The formation of a fluorescent screen by the photo-curing process using a shadow mask is accompanied by problems in that there is a higher initial cost, time- and labor-consuming recovery of extra phosphor resulting in the course of formation of the fluorescent screen is necessary and the phosphor is substantially lost. Accordingly, a printing process has recently been developed for the formation of a phosphor coating having a uniform thickness and little roughness on the surface of the phosphor coating and hence suited for a high-resolution CRT. According to this process, a CRT panel (front glass plate) is printed either directly or indirectly with a paste composed of a phosphor, an organic resin binder and a solvent and the binder resin component in the paste is thereafter burnt away to form a fluorescent screen. This process is industrially advantageous, because compared to the conventional processes for the formation of a phosphor coating the initial cost is lower, the loss of the phosphor is smaller and the material cost can hence be reduced, and the productivity is higher. As known phosphor pastes usable for this process, reference may be made to Japanese Patent Publication No. 23231/1986 and Japanese Patent Laid-Open No. 213778/1984.
As resins useful in these phosphor pastes, there are cellulose resins and vinyl alcohol resins. They however all require a high firing temperature and without exception also leave carbon as a residue. The physical properties of a phosphor coating formed from such a phosphor paste are thus poor, in particular, the lifetime of the resultant CRT is short. In addition, black spots are formed on the surface of the phosphor coating when the CRT is rendered luminous, so that the resolution of the CRT is lowered and the picture quality is deteriorated. In particular, the use of a resin of natural origin, e.g., cellulose as a binder, tends to result in the mixing of unexpected ionic impurities which may impair the characteristics of the phosphor or in the remaining of impurities which may deteriorate the burning-away property, since it is difficult to purify such a resin. It is therefore impossible to form a phosphor coating of high resolution from a phosphor paste which contains such a resin as a binder. As phosphor pastes said to have improved this problem, phosphor pastes making use of a synthetic resin of good burning-away property, for example, an alkyd resin or phenol resin have been proposed. These resins are however oil-soluble and have insufficient compatibility with a phosphor which is basically hydrophilic, whereby the dispersion of the phosphor would be poor in pastes to be formed. In other words, a coating formed by the printing process from such a phosphor paste is barely imparted with high resolution because it contains pinholes, the coating has poor levelling, and the phosphor is not coated evenly, therefore the uniform light-emitting property is poor.
Developed as phosphor pastes free of these inconvenience are those added with a compound which contain highly-ionic functional groups having compatibility with the phosphor, for example, phosphoric groups or nitrogen-containing functional groups. These phosphor pastes however have poor firing characteristics, thereby making it difficult to obtain from them a phosphor coating of high resolution and good picture quality.
For the production of color fluorescent screens, it is necessary to print a phosphor in a pattern such as stripes or dots. It is however difficult to print such a pattern in a desired size and with good accuracy whichever conventional phosphor paste is used. The primary reasons for this problem are that the phosphor pastes have insufficient thixotropy and the stripe or dot pattern once formed is allowed to flow to result in deformation.